Víctor R. Cueto

Migration Timing and Wintering Areas of Three Species of Flycatchers (Tyrannus) Breeding in the Great Plains of North America

ABSTRACT. Descriptions of intra- and interspecific variation in migratory patterns of closely related species are rare yet valuable because they can help assess how differences in ecology and life-history strategies drive the evolution of migration. We report data on timing and location of migration routes and wintering areas, and on migratory speed and phenology, of Eastern Kingbirds (Tyrannus tyrannus) from Nebraska and Oklahoma and of Western Kingbirds (T. verticalis) and Scissor-tailed Flycatchers (T. forficatus) from Oklahoma. Eastern Kingbirds primarily departed the breeding site in September, migrating to the Amazon Basin (Bolivia and Brazil), >6,400 km from their breeding site, then used a second wintering site in northwestern South America (Colombia, Ecuador, and Peru) before returning to the breeding site in April. Western Kingbirds left Oklahoma in late July, migrating >1,400 km to northwestern Mexico, then to central Mexico and finally to Central America before returning to Oklahoma in April. Scissor-tailed Flycatchers departed Oklahoma mainly in mid-October, migrating to Central America (El Salvador, Guatemala, Honduras, and Nicaragua), ∼2,600 km from the breeding site, remaining there until early April before returning to Oklahoma. Timing of migration appears to be tightly linked to molt. Early departure of Western Kingbirds from the breeding site appears to be timed so that they molt in the Sonoran Desert region during the monsoon, whereas Scissor-tailed Flycatchers remain at their breeding site to complete molt in late summer, when insect prey are abundant. Eastern Kingbirds delay molt until reaching South America where, possibly, abundant fruit supports molt.

SEED PREFERENCES IN SPARROW SPECIES OF THE MONTE DESERT, ARGENTINA: IMPLICATIONS FOR SEED-GRANIVORE INTERACTIONS

Abstract We assessed seed preference in four species of sparrows that are common to the central Monte desert of Argentina. Our hypotheses are that (1) sparrows always prefer grass over forb seeds and (2) different species of sparrows show the same preferences when consuming seeds. We presented seeds from eight species of plants (four grasses and four forbs) in both choice and nonchoice experiments. In general, sparrow species preferred grass over forb seeds, though one of them (Rufous-collared Sparrow [Zonotrichia capensis]) included one forb species (Parthenium hysterophorus) among its preferred seed items. Among grass seeds, sparrows avoided or showed less preference for the tiny seeds of Sporobolus cryptandrus, which suggests that seed weight may affect seed choice among grasses. Avoidance of most forb species, by contrast, could not be associated with seed size, probably because of the presence of unpalatable or toxic compounds in forb seeds. Experimental evidence suggests that the main feeding pressure of sparrows is clearly directed to grass seeds, which coincides with reported information on bird diet. Results support assertions that sparrows can cause a negative effect on the composition and abundance of grass seeds in the soil seed-bank, a necessary condition for birds in playing a significant role on grass population dynamics in the central Monte desert of Argentina. Preferencias por Semillas de los Emberízidos del Desierto del Monte, Argentina: Implicancias para la Interacción Semillas-granívoros

Long-Distance Bird Migration within South America Revealed by Light-Level Geolocators

ABSTRACT. Little is known about the timing of migration, migration routes, and migratory connectivity of most of the >230 species of birds that breed at south temperate latitudes of South America and then migrate toward the tropics to overwinter. We used light-level geolocators to track the migration of 3 male and 3 female Fork-tailed Flycatchers (Tyrannus savana) captured on their breeding territories in Argentina. All birds initiated fall migration between late January and late February, and migrated 45 to 66 km day-1 in a northwesterly direction through central South America to either one or two wintering areas. Five individuals first spent several weeks (in April and May) in western Amazonia (mainly Peru, northwestern Brazil, and southern Colombia) before moving east to spend the rest of the non-breeding season in central Venezuela and northern Brazil. One individual occupied primarily one wintering area in eastern Colombia, northwestern Brazil, and southwestern Venezuela. Fall migration took approximately 7–12 weeks to complete and covered a distance of 2,888–4,105 km. We did not analyze spring migration data because of broad overlap with the austral spring equinox. These results are the first data on wintering locations, migration timing, and routes of individual migrant passerine birds that breed in South America. Given the general lack of similar data for practically all migratory birds that breed in South America, geolocator technology has the potential to revolutionize our understanding of how birds migrate—and the threats they face—on South Americas rapidly changing landscape.

Bird assemblages of protected and exploited coastal woodlands in east-central Argentina

Abstract We surveyed vegetation and bird assemblages in protected and exploited woodlands in the warm temperate, coastal woodlands of Buenos Aires Province, Argentina. Exploited woodland experienced selective logging of Celtis tala until at least 1960; presently it is used for sheltering domestic livestock. Vertical structure and floristic composition were simpler in exploited than in protected woodland. Likewise, avian density and species richness were lower in exploited woodland. Viewing the avifaunas from a guild perspective, we found the insectivore guild and frugivore-insectivore guild differed substantially between protected and exploited woodlands. The bird densities of these guilds were higher in protected woodland, and the species richness of the insectivore guild was lower in the exploited woodland. Apparently the insectivore guild responded primarily to structural differences, whereas the frugivore-insectivore guild may have responded more to differences in floristic composition. The granivore guild also differed between the two woodlands, but primarily because of changes in the density of the Rufous-collared Sparrow (Zonotrichia capensis). This bird species was more abundant in the protected woodland.

The potential for comparative research across New World bird migration systems

For a migratory bird, the costs and benefits of utilizing a given migratory strategy vary according to the biotic (e.g., physiology) and abiotic (e.g., weather) constraints it experiences throughout the year. In the New World, closely related species migrate to breeding grounds located across a wide range of latitudes, from northern North America to southern South America. Because the ultimate goal of a bird on spring migration is to successfully arrive on the breeding grounds in a timely manner, events that occur during the breeding season (e.g., amount of time available to breed) could affect, through selection pressures, the behavior of birds on spring migration. Variation across north temperate, tropical, and south temperate latitudes in breeding strategies, breeding season length, and availability of food during the breeding season has been well documented in various bird species. Thus, such factors as migratory strategies, risk of mortality on migration, and effects of climate change on migratory patterns may also vary predictably, depending on the latitude, both north and south of the Equator, at which a migratory population breeds. Comparing such patterns across the New World, using interdisciplinary approaches and the latest in technological advances, holds promise for better understanding how migratory birds accomplish these spectacular journeys.

Selection of Foraging Sites by Desert Granivorous Birds: Vegetation Structure, Seed Availability, Species-Specific Foraging Tactics, and Spatial Scale

Abstract Seed availability in the algarrobal of the Monte desert, Argentina, as in other deserts, is highly heterogeneous at small scales and is associated with vegetation structure. Granivores are expected to use space selectively (within the capacities of their foraging techniques), resulting in a heterogeneous effect on the seed bank. First, we describe the foraging repertoire of granivorous birds in the algarrobal to develop predictions about their expected use of space. Although the granivory guild as a whole tracked the temporal availability of seeds, species within the guild differed in foraging behavior and seasonal changes. We assessed selection of space by foraging birds through a two-scale bird-centered analysis, comparing the distributions of used and available sites. Granivorous birds, considered as a guild, used the whole range of available microsites, though as a result of an aggregation of contrasting partial patterns. Microsites with more cover of shrubs, grasses, and litter were preferred for predispersal consumption, which is consistent with the frequent technique of attacking grasses from low woody perches. By contrast, use of microsites for postdispersal consumption did not differ from random, which suggests that there are no safe microsites for seeds in this habitat. At a larger scale not particularly related to heterogeneity in food availability, a selective pattern was clearer: birds avoided mesosites with low shrub and litter cover, far from trees. In conclusion, patterns are not straightforward and depend on considerations of spatiotemporal scale and species-specific characteristics. Even when predictions seemed clear-cut and were tested with proper scale and target-group, granivorous birds did not select space according to environmental clues of food abundance.

FORAGING BEHAVIOR AND MICROHABITAT USE OF BIRDS INHABITING COASTAL WOODLANDS IN EASTCENTRAL ARGENTINA

Abstract We examined foraging behavior and microhabitat use of four passerine bird species inhabiting an old growth coastal woodland in Buenos Aires Province, Argentina. Based on foraging maneuvers, we identified two groups: (1) nonaerial foragers formed by Tropical Parulas (Parula pitiayumi) and Masked Gnatcatchers (Polioptila dumicola) that hopped on branches and twigs while searching and gleaning prey from the nearby foliage, and (2) aerial foragers formed by White-crested Tyrannulets (Serpophaga subcristata) and Small-billed Elaenias (Elaenia parvirostris) that searched for and detected prey while perched, and captured it from foliage or in the air. Nonaerial foragers found prey primarily in the canopy while aerial foragers captured prey at all heights in the woodland. Tree species selection was similar among the four species. All bird species disproportionately foraged in Scutia buxifolia and avoided the use of Ligustrum lucidum trees. Our results indicate that these bird species differed in microhabitat selection in old growth coastal woodland, and that nonaerial foragers were more sensitive to foliage architecture and foliage height distribution than aerial foragers. These results illustrate the importance of woodland logging to bird densities, and provide basic information for effective management. RESUMEN.—Nosotros estudiamos el comportamiento de alimentación y el uso de microhábitats de cuatro especies passeriformes que habitan bosques costeros maduros de la Provincia de Buenos Aires, Argentina. Considerando las maniobras de alimentación encontramos dos grupos: (1) las recolectoras, formado por Pitiayumí (Parula pitiayumi) y Tacuarita Azul (Polioptila dumicola), las cuales saltando por las ramas y ramitas buscan y recolectan presas desde el follaje cercano, y (2) las cazadoras por revoloteo, formado por Piojito Común (Serpophaga subcristata) y Fiofío Pico Corto (Elaenia parvirostris), que buscan y detectan presas desde una percha y, usando tácticas aéreas, capturan las presas desde el follaje o en el aire. Las especies recolectoras capturan sus presas principalmente en el dosel del bosque, mientras que las cazadoras por revoloteo se alimentan en todas las alturas. El uso de las especies arbóreas del bosque fue similar para las cuatro especies de aves. Todas seleccionaron alimentarse en árboles de Scutia buxifolia y evitaron usar los de Ligustrum lucidum. En general, las especies de aves difieren en su selección de los microhábitats en el bosque costero maduro; las especies recolectoras fueron más afectadas por la arquitectura del follaje y la abundancia de cobertura en altura que las especies cazadoras por revoloteo. Los resultados obtenidos ayudan a comprender los efectos del talado del bosque sobre la densidad de aves, y proveen información básica para su efectivo manejo.

Which food patches are worth exploring? Foraging desert birds do not follow environmental indicators of seed abundance at small scales: a field experiment

Consumers should show strong spatial preferences when foraging in environments where food availability is highly heterogeneous and predictable from its correlation with informative environmental features. This is the case for postdispersal granivores in most arid areas, where soil seed bank abundance and composition associates persistently with vegetation structure at small scales (e.g., decimeters to meters). We analysed seasonal single-seed removal by granivorous birds from 300 experimental devices in the algarrobal of the central Monte desert. Spatial selectivity was analysed by comparing the structural characteristics of used vs. available microhabitats and evaluated against bottom-up and top-down hypotheses based on our previous knowledge on local seed bank abundance, composition and dynamics. Seed removal, which showed its expected seasonal variability, was also explored for spatial autocorrelation and environmental dependencies at bigger scales. Postdispersal granivorous birds were less selective in their use of foraging space than expected if patch appearance were providing them useful information to guide their search for profitable foraging patches. No kind of microhabitat, as defined by their vegetation and soil structure, was safe from bird exploration. The only consistent selective pattern at this scale was closer to a top-down spatial effect by birds, i.e., a cause (and not a consequence) of the seed bank dynamics. Bigger spatial scales proved more relevant to describe heterogeneity in the use of foraging patches in this habitat. Closeness to tall trees, probably related to bird territoriality and reproduction or to their perception of predation risk, seems to determine a first level of selection that defines explorable space, and then microhabitat openness exerts an influence on which patches are effectively exploited (or more frequently explored) among those accessible.

Migratory timing, rate, routes and wintering areas of White-crested Elaenia (Elaenia albiceps chilensis), a key seed disperser for Patagonian forest regeneration

Migratory animals often play key ecological roles within the communities they visit throughout their annual journeys. As a consequence of the links between biomes mediated by migrants, changes in one biome could affect remote areas in unpredictable ways. Migratory routes and timing of most Neotropical austral migrants, which breed at south temperate latitudes of South America and overwinter closer to or within tropical latitudes of South America, have yet to be described in detail. As a result, our understanding about how these birds provide links between South American biomes is almost non-existent. White-crested Elaenia (Elaenia albiceps chilensis) is a long-distance austral migrant that breeds in the Patagonian Forest biome and overwinters in tropical South America. Because this small flycatcher plays a key role in the regeneration of this ecosystem, our objective was to describe the annual cycle of White-crested elaenias to evaluate the degree of migratory connectivity between breeding and wintering areas and therefore to determine if there are specific biomes of northern South America linked by elaenias to Patagonian forests. Fifteen individuals were successfully tracked throughout a complete migration cycle using miniature light-level geolocators. All individuals resided and moved through the same general regions. During fall (March-April-May), elaenias were located in the Caatinga and the Atlantic Forest biomes, from Rio de Janeiro to the region near Salvador da Bahia, Brazil. During winter (June-July-Aug.), birds were located further inland, within the Cerrado biome. Birds used three different routes during fall migration. Our results indicate that some individuals use a direct route, flying between 500–600 km/day, crossing desert and grasslands, while others took a detour, flying 100–200 km/day through forested areas with refueling opportunities. All birds used the Yunga forest during spring migration, with ten out of 15 individuals showing a clear counterclockwise loop trajectories throughout their annual cycle. None of the elaenias passed through Amazonia, traveled to western South America or crossed the Equator. Eleanias exhibited a high migratory connectivity between breeding area in Patagonian Forests and winter areas, Atlantic Forest and Cerrado. Our results suggest that Patagonian Forests could be strongly impacted by changes in those biomes or in the Yungas.

Interannual Variation In Foraging Behavior of White-Crested Elaenia (Elaenia albiceps chilensis) In A Northern Patagonian Forest

ABSTRACT Avian foraging behavior may vary among years because of changes in the abundance and availability of prey. We therefore aimed to study the foraging behavior of White-crested Elaenia (Elaenia albiceps chilensis), a Neotropical austral migrant, in relation to interannual variation in its food resources in a mixed forest of Patagonia. During two summers of contrasting rainfall patterns, we recorded foraging maneuvers, substrate use, and use of forest microhabitat (plant species and heights) where birds capture their prey. We also evaluated the abundance of the main food items consumed by this species: ripe fruit and arthropods. The first year was very rainy, when we recorded a lower supply of ripe fruits than in the second year, whereas the abundance of arthropods was similar throughout the study. During both years, the most frequent behavior of White-crested Elaenia was searching for food items while perched, capturing them from foliage using gleaning and sally-hovering maneuvers. Nevertheless, this species notably modified the use of foraging microhabitat between years. During the first year, elaenias used the canopy of the forest and foraged mainly in Nothofagus dombeyi trees, whereas in the second year elaenias used the entire profile of vegetation height and increased prey capture in shrubs of Aristotelia chilensis. Our results demonstrate the behavioral flexibility of this Neotropical austral migrant species and highlight the importance of considering the spatio-temporal variation of resources when evaluate the foraging behavior of birds.